1. The problem statement, all variables and given/known data Hello everyone! To simplify, I have a system whose transfer function looks like this: G = (s^2-1) / (s^4-s^2-1) It's unstable and I have to find a way to stabilize it using PID controller and/or lead/lag compensators. 2. Relevant equations 3. The attempt at a solution So far I tried a simple PID controller (Kp + Ki/s + Kd*s), a PID controller with low pass filter (Kp + Ki/s + Kd*s/(1+N*Kd*s), a PID controller with multiple derivatives and integrators (P*I^m*D^n) but no luck. I'm using matlab to solve it. Root locus attached. Could you give me any hints? Thank you.